This commit changes many things:
* Most interface-related operations are moved into an optimization
pass for more modularity. IR construction creates pseudo-calls which
are lowered in this pass.
* Type codes are assigned in this interface lowering pass, after DCE.
* Type codes are sorted by usage: types more often used in type
asserts are assigned lower numbers to ease jump table construction
during machine code generation.
* Interface assertions are optimized: they are replaced by constant
false, comparison against a constant, or a typeswitch with only
concrete types in the general case.
* Interface calls are replaced with unreachable, direct calls, or a
concrete type switch with direct calls depending on the number of
implementing types. This hopefully makes some interface patterns
zero-cost.
These changes lead to a ~0.5K reduction in code size on Cortex-M for
testdata/interface.go. It appears that a major cause for this is the
replacement of function pointers with direct calls, which are far more
susceptible to optimization. Also, not having a fixed global array of
function pointers greatly helps dead code elimination.
This change also makes future optimizations easier, like optimizations
on interface value comparisons.
A function on a type that is put in an interface must not be assumed to
not need a context pointer when it isn't directly put in a function
pointer, because the interface call side won't know this and pass an
extra parameter with that extra function pointer.
This is usually not a problem but WebAssembly has strict checks on
function signatures so this resulted in an error when running
src/examples/test/test.go.
This is one step towards removing unnecessary special casts in most
cases. It is also part of removing as much magic as possible from the
compiler (the pragma is explicit, the special name is not).
